The present invention relates to an electrical current transducer comprising a magnetic circuit with a magnetic core having a gap. The present invention is in particular related to an electrical current transducer with a wound magnetic core and a grounding device.
Many conventional current transducers comprise a magnetic core made of material with a high magnetic permeability and a magnetic field sensor, such as a Hall effect sensor, positioned in a gap formed in the magnetic core. A primary conductor extending through a central passage of the magnetic circuit generates a magnetic field that is picked-up by the magnetic core. The magnetic field flows across the gap and the magnetic field detector positioned therein. Since the gap represents a zone of low magnetic permeability and thus has an important effect on the magnetic field lines, it is important to accurately control the width of the gap in order to ensure accurate and reliable measurement of the electrical current to be measured.
It is also important to reduce losses in the transducer, in particular losses due to the formation of eddy currents in the magnetic core and to avoid magnetic saturation along any section of the magnetic core. The use of stacked laminated sheets to reduce eddy currents is well-known. A known means of forming a stacked multilayer magnetic circuit is by winding a thin band or strip of magnetic material to form an annular wound core. It is known to provide wound cores with air gaps, whereby the manufacturing process consists of first winding an annular toroidal core, subsequently applying resin to the core to hold the concentric layers of strip material and subsequently machining a gap radially through a section of the winding. Once the resin has been applied, annealing of the material of the wound magnetic core is difficult or no longer possible in view of the high temperatures required for the annealing process.
Working of materials with high magnetic permeabilities can affect their magnetic properties, in particular by reducing their magnetic permeability and thus adversely affecting the magnetic performance of the magnetic circuit.
To avoid electrical discharge and ensure an accurate measurement of the current flowing in the primary conductor, the magnetic core is often connected to ground. Typically this is done by soldering or even welding an electrical conductor, which is configured to be connected to ground, to the magnetic core. The ground connection may also be achieved by positioning an electrical conductor plate in contact with the magnetic core without changing the structure of the magnetic core. A method of connecting a magnetic core to ground without changing the structure of the magnetic core is for instance shown in JP 4250365, in which a grounding element, such as a circuit board, is positioned against a surface of the magnetic core to produce a capacitive coupling between the magnetic core and the grounding element. A drawback of this design is that the grounding element is not held in position in relation to the magnetic core and thus the connection between the grounding element and the magnetic core is not stable and not very resistant in harsh environments. Another disadvantage concerns the various parts that are present during the assembly, and the fragility of such a construction.
The gap length of a magnetic circuit may vary due to thermal and mechanical forces. It is known to stabilize the size of the gap by means of an element fixed to the magnetic core. In JP 2 601 297 the air gap of an annular wound magnetic core is fixed by means of a T-shaped element having a portion partially inserted in the air gap from the outer radial side of the magnetic circuit, the insert being held in place by means of a band wound around the magnetic circuit and the insert. A drawback of this design is that the insert partially engages in the air gap and thus limits the space for insertion of a magnetic field sensor. Moreover, the insert only engages the outer peripheral layers of the magnetic circuit and thus does not prevent variation of the size of the air gap of the inner radial layers of the magnetic circuit, in particular variations due to thermal forces that the resin binding the layers cannot entirely prevent. Also, heat treatment of the magnetic circuit after application of the resin is either not possible or at best limited. The position of the insert from the outer radial periphery of the magnetic circuit also increases the size of the magnetic circuit.
In EP2224461, a magnetic circuit with a wound core and a bridging element welded on a lateral side of the core and spanning across the air gap, is disclosed. This offers a compact configuration with stable fixing of the air gap. The welded connection between the bridging element and the magnetic core may be used for grounding the magnetic core via the bridging element and fixing pins thereof, however the welded connection between the bridging element and the magnetic core may reduce the magnetic permeability of the magnetic core in the welding zone. Moreover, in particularly harsh environments, such as those found in automotive applications, the weld connections subjected to thermal and mechanical shock as well as a corrosive environment may deteriorate over time.